The validity of radioimmunoassay determinations of serum digoxin and digitoxin concentrations to diagnose digitalis toxicity is at present controversial. Since the radioimmunoassay measures the total amount of digitalis compounds in the serum, metabolic products are quantitated as though they were the parent compounds. Some of these metabolites have been shown to be relatively inactive while other metabolites are more active than the parent compounds. The presence of these metabolites may be important in the interpretation of serum RIA levels for digoxin and digitoxin. The proposed research will be directed toward the development of a method to isolate and quantitate digoxin, digitoxin and their metabolites in the serum, urine and feces of patients taking these drugs. This method involves the isolation of the parent compounds and their metabolites by high-pressure liquid chromatography and their subsequent quantitation by radioimmunoassay. These metabolite profiles will be correlated with the clinical findings in an attempt to establish more valid criteria for the determination of digitalis toxicity by radioimmunoassay. Long range plans include the use of this method to determine if certain pathophysiological conditions or drug regimens may affect the pharmacokinetics of digoxin and digitoxin in man.